docs: def groupletters(x, _low=lower_function):
      """
      Double all characters, making doubled letters lowercase.
  
      Parameters
      ----------
      x : str
  
      Returns
      -------
      str
  
      Examples
      --------
  
          >>> groupletters("Apple")
          {u}'aAppppllee'
  
      """
      return "".join(ichain.from_iterable((_low(y), y) for y in x))